Yes, Facebook supplies a list of image sizes via their Graph API. My plugin uses the photo that best represents the size being used. By default the height and width of the thumbnail is 130px by 130px, so a function is used to determine which image is closest to that size and uses it. This is done for performance reasons. If you know CSS, you can play around with this and get a bigger, higher quality image that you can shrink down. To do this, use the height and widget attribute in the shortcode:
[fb_album width=900 height=900 … ]
Then you can use background-size and width/height in your CSS to get the thumbnail to the size you like.
I’ll look into this issue more in the newest version I’m working on now and put in a shortcode option to use lower quality thumbnails for better performance and higher quality thumbnails for better look.